1. Field of the Invention
The present invention relates to an organic electro-luminescence display device, and more particularly, to an organic electro-luminescence display device and a method of driving the same that is adaptive for reducing power consumption by removing an unnecessary current as well as for improving a uniformity of a display screen.
2. Description of the Related Art
In recently, there has been developed various flat panel displays with a reduced weight and bulk that are free from the disadvantage of a cathode ray tube CRT. Such flat panel displays include a liquid crystal display LCD, a field emission display FED, a plasma display panel PDP, and an electro-luminescence (hereinafter, referred to as an EL) display devices.
The structure and fabricating process of the PDP among these is relatively simple. Thus, the PDP is most advantageous to be made large-sized, but has disadvantages that the light emission efficiency and brightness thereof are low and its power consumption is high.
The LCD is used as a display device of a notebook computer, the demand for it is gradually increased. However, the LCD is difficult to be made large-sized because of using a semiconductor process, and the LCD requires a separate light source because it is not a self-luminous device. Accordingly, the LCD has a disadvantage that the power consumption is high due to the separate light source. Further, the LCD has a disadvantage that there is a high optical loss caused by optical devices such as a polarizing filter, a prism sheet and a diffusion panel, and its viewing angle is narrow.
The EL display device is generally classified into an inorganic EL display device and an organic EL display device. The EL display device has an advantage that its response speed is fast, its light-emission efficiency and brightness are high, and it has wide viewing angle. The organic EL display device can display a picture in a high brightness of several ten thousands [cd/m 2] with a voltage of about 10[V] and has been applied to most of EL display devices, which are commonly used.
In a unit element of an organic EL display device, as shown in FIG. 1, an anode 2 is formed of a transparent conductive material on a substrate 1; and a hole injection layer 3, a light-emitting layer 4 made of an organic material and a cathode 15 made of a metal having a low work function are disposed thereon. If an electric field is applied between the anode 2 and the cathode 5, then holes within the hole injection layer 3 and electrons within the metal are progressed into the light-emitting layer 4 to combine each other in the light-emitting layer 4. Then, a phosphorous material within the light-emitting layer 4 is excited and transited to thereby generate a visible light. In this case, the brightness is in proportion to a current between the anode 2 and the cathode 5.
Such an organic EL display device is classified into a passive type and an active type.
FIG. 2 is a circuit diagram showing equivalently a portion of the passive type organic EL display device, and FIG. 3 is a waveform diagram showing waveforms of a scan signal and a data signal in the passive type organic EL display device.
Referring to FIGS. 2 and 3, the passive type EL display device includes an organic EL elements OLED arranged at intersections between both a plurality data lines D1 to D3 and a plurality of scan lines S1 to S3, which cross each other, and both a plurality data lines D1 to D3 and a plurality of scan lines S1 to S3, which cross each other.
The data lines D1 to D3 are connected to an anode of the organic EL element OLED to supply a data current Id to the anode of the organic EL element OLED.
The scan lines S1 to S3 are connected to a cathode of the organic EL element OLED to supply scan pulses SP1 to SP3, synchronized with the data current Id, to the cathode of the organic EL element OLED.
The organic EL elements OLED emit light in proportion to a current flowing between the anode 2 and cathode 5 during a display period DT when the scan pulses SP1 to SP3 are applied thereto. The organic EL elements OLED are charged with current during a response time RT delayed by a resistance component of the data lines D1 to D3 and a capacitance existed in the organic EL elements OLED, so that there is a problem of a low response speed and a low brightness. In order to compensate a low response speed of the organic EL elements OLED, it is on a trend that a pre-charge period PCHA is provide in non-display periods DCHA and PCHA between the display period DT and the display period DT as shown in FIG. 4 and the organic EL devices OLED are pre-charged during the pre-charge period PCHA.
However, in the related art pre-charge drive method, a maximum data current is supplied during the pre-charge period PCHA irrespective of gray level value of data applied to the organic EL elements OLED via the data lines D1 to D3 during the display period DT, and then a data current correspondence to the data gray level value is supplied to the organic EL elements OLED during the display period DT. Accordingly, if a data current of low gray level is supplied via the data lines D1 to D3 to the organic EL elements OLED during the display period DT, then an overshoot is generated and a response time of the organic EL elements OLED is delayed. In addition, the organic EL elements OLED are over-charged due to an unnecessary current supplied to the organic EL elements OLED via the data lines D1 to D3 during the pre-charge period PCHA in the low gray level. Accordingly, there is a problem that power consumption is increased in the organic EL display device.